How to check if file has valid JSON syntax in Powershell - json

Question

I am trying to write a powershell script that reads a file and prints "true" if it is a valid JSON file. I am using Powershell v3.0 and this is what I have right now :
$text = Get-Content .\filename.txt -Raw
$powershellRepresentation = $text | ConvertFrom-Json
How do I check the return code? I mean I want something like this :
if(file not a JSON file){
Write-Host "not JSON"
}
else{
Write-Host "True"
}

Answer 1

UPDATE 2021: PowerShell 6 and newer versions
PowerShell 6 brings a brand new Test-Json cmdlet. Here is the reference.
You can simply pass the raw file content directly to the Test-Json cmdlet.
$text = Get-Content .\filename.txt -Raw
if ($text | Test-Json) {
$powershellRepresentation = ConvertFrom-Json $text -ErrorAction Stop;
Write-Host "Provided text has been correctly parsed to JSON";
} else {
Write-Host "Provided text is not a valid JSON string";
}
PowerShell 5 and earlier versions
There is no Test-Json cmdlet in these versions, so the best way is to put your ConvertFrom-Json cmdlet inside a try ... catch block
try {
$powershellRepresentation = ConvertFrom-Json $text -ErrorAction Stop;
$validJson = $true;
} catch {
$validJson = $false;
}
if ($validJson) {
Write-Host "Provided text has been correctly parsed to JSON";
} else {
Write-Host "Provided text is not a valid JSON string";
}

Answer 2

If you encounter this question and can use PowerShell 6 or later, there is now a Test-Json cmdlet. It can also not just validate that it's valid JSON, but that it conforms to a particular JSON schema using the -Schema param.
Example
$isValid = Get-Content .\filename.txt -Raw | Test-Json
if($isValid){
Write-Host "not JSON"
}
else{
Write-Host "True"
}
ARM Template Warning
A note for users looking to validate an ARM template via -Schema (I can't imagine a more perfect use case). At the time of writing, there are one or more bugs in the underlying library Test-Json uses for validation, NJsonSchema, and it is not possible to validate an ARM template.
GitHub Issues
PowerShell Issue #9560
NJsonSchema Issue #588

Answer 3

I don't think that it exists an other solution than catching the exception using ConvertFrom-Json.

Answer 4

ConvertFrom-JSON would work but only for a JSON object < 2MB in size.
For higher you can use JavaScriptSerializer class
try
{
$jsser = New-Object System.Web.Script.Serialization.JavaScriptSerializer
$jsser.MaxJsonLength = $jsser.MaxJsonLength * 10
$jsser.RecursionLimit = 99
$outObject = $jsser.DeserializeObject($json)
}
catch
{
Write-Host "Error converting $text to JSON"
}

Answer 5

ConvertFrom-Json shd be the appropriate way to go.
Test-Json unfortunately has alot of known unsupported JSON Types.
I.E. it cannot parse Json-Arrays or Primitives properly leading to falsely assuming it has wrong JSON-Syntax.